1. Field of the Invention
The invention relates to an erasable image forming material.
2. Description of the Related Art
Forest conservation is an essential requirement to maintain the terrestrial environment and suppress the greenhouse effect caused by CO2. In order to minimize additional tree trimming and to keep balance with forest regeneration including tree planting, it is important how to utilize the existing paper resources efficiently.
Currently, paper resources are “recycled” by recovering paper fibers from used paper through a deinking step of removing image forming materials printed on the used paper, remaking paper fibers to manufacture recycled paper with low paper quality, and using the recycled paper according to the purpose. Thus, problems of a high cost of the deinking step and possibility of new environmental pollution by waste fluid treatment are pointed out.
On the other hand, “reuse” of a hard copy has been put into practice through erasure of images, for example, by using an eraser for pencil images and a correcting fluid for ink images. Here, the concept of “reuse” in which a paper sheet is repeatedly used for the same purpose while preventing degradation of paper quality as much as possible is different from the concept of “recycling” in which a paper sheet with degraded quality is used for other purposes. Now, the “reuse” can be said to be more important concept from a viewpoint of conservation of paper resources. Recently, a rewritable paper has been proposed, which is a special paper intended to reuse hard copy paper. Use of the rewritable paper technology enables the paper to be “reused” many times. However, the rewritable paper is a special paper which can be “reused” but cannot be “recycled”. Accordingly, the rewritable paper is a defective technique from the viewpoint of protection of paper resources.
The present inventors have paid their attention to a phenomenon caused by a system of a color former and a developer that a colored state is realized when interaction between the color former and the developer is increased and an erased state is realized when the interaction is decreased. Thus, the inventors have proposed, as paper reuse techniques, image forming materials made of a composition system comprising a color former and a developer to which an erasing agent capable of capturing the developer is added. The image forming materials can exhibit stably a colored state around room temperature and can retain an erased state for a long time at practical temperatures by treatment with heat or a solvent. The inventors have also proposed image erasing processes and image erasing apparatuses using the image forming materials. These image forming materials have advantages of highly stable colored and erased states of the images, highly safety in view of materials, applicability to electrophotography toners, liquid inks, ink ribbons and writing instruments, and feasibility of large-scale erasure treatment, which cannot be realized so far. Since the image forming materials can be recycled after the reuse steps, the efficiency of use of paper resources can be remarkably improved.
The present inventors have further found through intensive studies on the improvement of the erasable image forming material that, in a case where a paper sheet is used as image recording medium, even an image forming material not containing an erasing agent can be erased by above two methods of treatment with heat or a solvent, because cellulose that is a constituting component of paper has an ability to capture the developer. Thus, the present inventors have also proposed an image forming material containing no erasing agent and an erasing method for the material. See, for example, JP-A 2000-284520 (KOKAI).
However, since the paper sheet takes the function of capturing the developer for the particular erasable image forming material, the erasure performance of the material is controlled by diffusion migration of the developer in the binder resin. Accordingly, in the above erasable image forming material, the property of the binder resin becomes one of factors to prevent the erasure performance from being improved.